In recent years, a catalyst or a filter for cleaning toxic substances has come to be requested with an enhancement of a concern to environmental preservation.
Nitrogen oxides generated from internal combustion engines such as an automobile engine or industrial plants are air pollutants harmful to a human organism or the environment, so that it is required to decrease nitrogen oxides. As a method for removing nitrogen oxides, there have been practically used a three way catalyst method for a gasoline engine and a selective catalytic reduction method using ammonia for industrial plants or the like.
Further, recently, a dark smoke particulate matter (PM) discharged from a diesel engine is particularly receiving attention as a substance that causes serious harm to a human organism. As a material which collects PM, a filter produced from a silicon-carbide-based fiber has been put to practical use. Further, there are carried out attempts to decrease PM combustion or nitrogen oxides by making a catalyst component be carried on a silicon-carbide-based fiber surface.
However, these environmental cleaning techniques have a variety of problems with regard to practical uses of these techniques. Concerning the cleaning of nitrogen oxides, when high-concentration oxygen is contained like a diesel engine exhaust gas, it is difficult to adopt the three way catalyst method since an oxidation reaction of a reducing agent (CO or hydrocarbon) preferentially occurs. Further, when the selective catalytic reduction method using ammonia is adopted, it is required to handle an ammonia gas having high toxicity and corrosivity. Concerning the PM, a current filter made of a silicon-carbide-based fiber has high collection effect, while it is impossible to remove PM combustion unless a catalyst component is carried on the fiber. Further, it does not have function of cleaning nitrogen oxides discharged together with PM.
Further, a silicon-carbide-based fiber is generally poor in alkali resistance so that a very small quantity of alkali metal or alkali earth metal contained in an exhaust gas causes corrosion. Further, when alkali metal or alkali earth metal as a catalyst component is carried on the fiber, a deterioration of the fiber due to corrosion becomes very remarkable.
It is estimated that the above corrosion deterioration can be improved by using a zirconia fiber excellent in alkali resistance in place of the silicon-carbide-based fiber. However, a current zirconia fiber is synthesized mainly according to a sol-gel method, as described in, for example, New Ceramic No. 8, pages 53 to 58, (1996). Its tensile strength is very low or 1 Gpa or lower and it is insufficient in practical strength.
Furthermore, for making a catalyst be carried on a fiber, it is preferred that the fiber has a larger specific surface area. However, the specific surface area of the current zirconia fiber is low or 10 m2/g or less.